In this application "frequency convertor" means a static convertor for transforming AC of one frequency to AC of another frequency. "Intermediate link convertor" means a frequency convertor having a rectifier and an inverter, which are joined by a DC link. "Direct convertor" means a frequency convertor without a DC intermediate link (often also called a "cycloconvertor").
It is known in the art that synchronous motors may be supplied by means of intermediate link convertors having a recifier and a machine-commutated inverter of variable frequency. At low speeds, the voltage of such machines is low. In order to obtain a safe commutation, special measures must be taken, for example, the current may be limited or reduced for each commutation. However, such current limiting techniques cause torque pulsations and result in a considerable reduction of the available torque, thereby reducing the efficiency of utilization of the machine. Moreover, in convertors of the above-mentioned type, at least four valves are operated to conduct simultaneously (two in the rectifier and two in the inverter), thereby causing a relatively high voltage drop and correspondingly high convertor losses.
Also, the invertor for an intermediate link convertor of a two-phase machine cannot be made as a standard convertor, for example a conventional three-phase bridge, but must be specially made, both with regard to its main circuits and its auxiliary circuits (e.g. protective, supervision, and control circuits). Thus, considerable economic and practical disadvantages are associated with such intermediate link convertors.
Accordingly, it is a primary object of the invention to provide a motor drive system of the above-described kind wherein, particularly at low speeds, the torque pulsations of the motor are considerably lower than for previously known apparatus.
It is another object of the invention to provide such drive equipment that efficiently utilizes a machine while maintaining low convertor losses.
A further object of the invention is to provide a drive system wherein simple standard convertors may be modified to a limited extent to supply a motor.